1. Field of Invention
The present invention relates to an apparatus and method of preparing a reference solution, and more particularly, to an apparatus and method of preparing a reference solution of a gaseous substance necessary for calibration of various measuring instruments used in measurement of the gaseous substance, such as radon (Rn-220) or a volatile substance, contained in seawater, subsurface water, surface water or the like.
2. Description of the Prior Art
Radon is generated from three types of naturally occurring radioactive series, uranium series (U-238), actinium series (U-235), and thorium series (Th-232), and three isotopes of radon occur in nature: Rn-222 (having a half-life of 3.82 days), Rn-219 (having a half-life of 3.96 seconds) and Rn-220 (having a half-life of 55.6 seconds). Among the three radon isotopes, Rn-222 having the longest half-life of 3.82 days is commonly referred to as radon.
Radon is a colorless and odorless inert gas such as helium (He) or neon (Ne) and is known as a very harmful substance to the human body when it is inhaled. International Agency for Research on Cancer (IARC) classifies radon along with asbestos as class one carcinogens, and Environment Protection Agency (EPA) classifies radon as a carcinogen after smoking.
In practice, a risk of radon is mostly caused by radon progenies rather than radon itself. Although there are eight types of radon progenies, when considering a generation rate and half-life, a representative nuclide dangerous to the human body includes Po-218, Po-214, Bi-214, Pb-214 and the like (National Council on Radiation Protection and Measurements, NCRP, 1988). All the radon progenies are heavy metals, have chemicophysical properties completely different from radon, and easily adsorbed to air suspended substances such as aerosol and dust.
It has been known that after radon occurring in the air and radon progenies adsorbed to suspended substances are introduced along with other gaseous substance into the human body when they are inhaled and accumulated on a lung wall, lung cancer is caused by the transformation of lung cells exposed to alpha particles generated during the decay of the radon and the radon progenies.
It has been known that a mean effective radiation exposure of radon to an ordinary human body is higher than that of total natural radioactivity except radon or medical treatment activity such as X-ray examination and also much higher than that caused by industrial activity such as in a nuclear power plant. The EPA reports that 5,000 to 20,000 persons die of lung cancer due to radon each year in the United States based on the calculations of scientists.
It has also been known that the concentration of radon and radon progenies occurring in the air mainly depends on the uranium content of soil distributed in the vicinity of us and about 80% of radon in the air originates from the surface layer of soil. Also, radon has a higher solubility in water than other inert gas. Therefore, subsurface water serving as a carrier of radon released from rock or fissure, with which the subsurface water comes into contact in subsurface flow paths, contains 1,000 to 10,000 times more radon than seawater or surface water such as river water.
Radon is receiving attention as a harmful substance to the human body and also in various fields including health science, environmentology, oceanography, climatology, and the like due to physicochemically stable inert properties, a high radon content in subsurface water, and the like. Recently, as interests in conservation, securing and development of subsurface water are increased, radon is being used as a very suitable tracer in connection studies of subsurface water and surface water. In addition, radon tends to be actively used in analysis of generating factors and countermeasure research of algal blooms, red tides and the like due to the subsurface water outflow to the sea or lake.
As radon receives attention in a variety of fields as described above, a device and method of measuring radon has been constantly developed. Typical methods of measuring radon developed so far include gamma spectroscopy of measuring radon progenies, Pb-214 and Bi-214, a Lucas cell method using scintillation cells, a liquid scintillation counting (LSC) method using a liquid scintillation solution, and the like. In order to precisely measure radionuclides or chemical components, calibration of a device used in the measurement should be preceded. The calibration of such a device is generally performed using a reference or standard substance necessary thereto.
The calibration standard substance should secure homogeneity and long-term stability and need matrix matching. However, unfortunately, no commercialized radon reference solution has been developed so far since radon has a half-life of 3.82 days, which is not suitable for being used as a standard substance in terms of long-term stability and matrix matching.
Because of these problems, a reference solution of radium (Ra-226, a half-life of 1,600 years), which is a parent nuclide of radon, is commercialized as an alternate substance of radon reference solution. However, such a radium standard substance is prepared by synthesizing a radium compound through complicated procedures and dissolving it in strong acid. Therefore, when the radium standard substance is used after being diluted, the radium standard substance has different components dissolved therein from an actual target object desirous to be measured, such as subsurface water, seawater, river water, or leachate, which increases uncertainty of measurement, resulting in inaccurate calibration of a device. In order to accurately perform indoor air pollution evaluation of radon, effects evaluation of subsurface water-surface water using a radon tracer, and the like, it is urgent to develop a radon reference solution with matrix matching with a target object to be measured.